Q. Describe properties and uses of different silicate glasses and also discuss the importance of annealing in the manufacture of glass.
OR
Discuss the composition, uses and types of glasses.
Ans. Glass is an amorphous, hard, brittle, transparent, super cooled liquid of infinite viscosity, obtained by fusing a mixture of a number of metallic silicates, most commonly of Na, K, Ca, viscosity, obtained by fusing a mixture. It possesses no sharp melting point, definite formula or crystalline structure. Within certain limits, it may be represented as XR2O.YMO.6SiO2, where R is an atom of mono valet alkali metal like Na, K, etc. M is an atom of a bivalent metal like Ca, Zn etc. X; and Y are whole numbers. Thus, approximately composition of ordinary glass is N2O.CaO.6SiO2.
Different silicate glasses
1. Soft lime or soft glass: The raw materials are silica, calcium carbonate and soft ash. Their approximate composition is Na2O.CaO.6SiO2. in some glasses, SiO2 may be replaced by Al2O3.B2O3.P2O3 etc. They melt easily resistant to water. They melt easily and hence, can be not worked easily.
Uses: They are widely used as window glasses, electric bulbs, place glasses, building blocks, and cheaper tableware's, where high temperature resistance and chemical stability are not required.
2. Potash lime or soft glass: The raw materials are silica.
Uses: Used for making chemical apparatus, combustion tubes, etc, which are to be used for heating operations.
3. Lead glass or flint glass: It is made by using lead oxide, instead of calcium oxide for fusing together with silica. For dense optical glasses as much as 80% of is incorporated. In addition, K2O is used, instead of sodium oxide.
Uses: Used in high quality table wares, optical purposes (like lenses etc.), neo sign tubing's, cathode ray tubes, electrical insulators and in art objects, because of their high lustre, high lead content glasses are used for extra dense optical glasses for windows and shields to protect from X-ray and gamma rays in medical and atomic energy field respectively.
4. Borosilicate glass or Pyrex glass: It is the most common of the hard glasses of commerce use. Such glasses contain virtually only silica boron, with a small amount of alumina and some alkali oxides.
Uses: Used as pipelines for corrosive liquids, gauge glasses, superior laboratory apparatus, kitchens wares, chemical plants, television tubes electrical insulators etc.
5. Alumina silicate glass: These are exceptionally high softening.
Uses: Used as for high pressure mercury discharge tubes, chemical combustion tubes, temperature certain domestic equipments etc.
6. 99.5% silica glass or virtuosi: It is produces by heating pure sand (silica) to its melting point.
Uses: It is used mainly for chemical plants. Chemical laboratory wares, electrical insulating materials in electrical heaters, furnaces etc.
7. Safety glass: It is made by taking two or three flat sheets of glass and in between them alternate thin layer of vinyl plastic is introduced and the whole is subjected to slight pressure. It is then heated till the glass layers and plastic layers into one another to give a sandwich. On cooling the glass becomes quite tough. When such a glass breaks, it does not fly into pieces. Since, the inner plastic layer tends to hold back the broken pieces of the glass.
Uses: It is used in automobile and aeroplane industries as wind shields etc.
8. Optical or Crookes glasses: Contain phosphorous and lead silicate, together with little cerium oxide.
Use: They are used for making lenses.
9. Polycrystalline glass or Pyroceram: It is produced by adding one or more nucleating agents to a special or conventional glass batch, which is then shaped into a desired form. The material is then subjected to a controlled heat treatment. Crystalline glass is not ductile, but has much greater impact strength than ordinary glass. It exhibits high strength and considerable hardness and can be formed and shaped into desired moulds.
Toughened glass: It is made by dipping articles still hot in an oil bath, so the same chilling take place. By doing so the outer layer of articles shrinks acquire a state of compression, while the inner layers are in a state of tension. Such a glass is more elastic and capable of withstanding material and thermal shocks. When such glass breaks it does not fly, but is reduced to fine powder.
Uses: It is used for window shields of fast moving vehicles, window shields of furnace, automatic doors and large cases etc.
Annealing: The manufactured glass articles are cooled very slowly and this process of slow cooling, which is used to reduce strain, is known as annealing. Each type of glass has its own critical temperature below its softening point as summarized in table for the lime and soda glass of Pyrex glass. In annealing the manufactured glass articles should be kept above critical temperature for a sufficiently long time and also should be cooled gradually and slowly. By the process of annealing, durable articles which can withstand shock and change of temperature can be made. It is, therefore necessary to anneal all types of glass, whether formed by machine or hard mould methods. For every types of glass, there is a definite annealing, temperature and if glass in maintained as near as possible to this temperature strain is rapidly removed.